What’s the Connection between Slow Processing Speed and ADHD?

Slow processing speed is very common in children who are diagnosed with ADHD. You may think of kids with ADHD as naturally quick thinking — “too” quick perhaps — but studies show that 60% of kids with ADHD display signs of slow processing speed. Slow processing speed and ADHD are definitely connected.

Consider the pace at which many kids with ADHD, Combined or Hyperactive/Impulsive Type do their homework. Some rush through it with little concern about accuracy, getting it done quickly but making many mistakes. While they do things quickly, they are inefficient and score lower on psychological measures of processing speed that define processing speed as the speed of completion of a task with reasonable accuracy. Other kids with ADHD, Inattentive Type tend to work very slowly, displaying signs of a sluggish cognitive tempo and sometimes visibly processing information slowly.

It is important to understand what psychologists mean by the term “processing speed.” Measurement of slow processing speed is not all about “speed” but is also about accuracy and consistency. Classic measures of processing speed take into account how many “easy” items are answered correctly in a set amount of time. Many of these measures penalize mistakes, taking points away on the simple tasks that constitute processing speed tests.

In simple terms, one can obtain a low score on measures of processing speed by completing a below average number of items or by completing an above average number of items but making many mistakes. This helps to explain some of the connection between slow processing speed and ADHD kids who “speed” through a task without regard to accuracy.

Some kids with ADHD are slower at tasks because they are not prepared or ready to act and are unable to complete as many items as their peers on tasks requiring fast processing. While some kids with ADHD, Inattentive Type can become distracted on even a one- to two-minute task, other children with ADHD may display slow processing speed that is related to slow reaction time. These kids tend to be slower overall at perceiving the stimuli used in the testing, not necessarily at the point of noticing it but in the time it takes between perception and action.

Even more perplexing is that children with ADHD appear to be capable of normal reaction time as measured on common tests of visual attention such as the Conners Continuous Performance Test (CPT) but tend to be more variable in their reaction time. Research shows that kids with ADHD can react at the same pace as their typically developing peers but show more variability over the course of time, perhaps going faster but sometimes slower. This is consistent with the observation that children with ADHD can be impulsive on some occasions but appear distracted or “zoned out” on others.

This suggests that variability in reaction time/processing speed may be the connection between ADHD and processing speed. Neuroscience research suggests that the biological cause is that the premotor and prefrontal circuits of the brain of ADHD kids may be slower in their state of preparedness to respond to external stimuli. Therefore for some kids with ADHD, slower processing occurs between observing and acting.

There is also evidence that many kids with ADHD are slower on the “acting” or output side of processing speed. ADHD kids are also observed to be slower on measures of skelemotor (movement) and oculomotor (looking at) skills. Interestingly, there is some support for this observation in the research on how kids with ADHD play video games, which suggests that, as much as they seem to be attentive and engaged in video games, they react more slowly than their typically developing peers.

The connection between slow processing speed and ADHD is clearly complex. Many teens and adults with ADHD identify themselves as having “faster brains,” yet acknowledge how common it is for them to go slower when distracted by their environment, making decisions, and doing tasks that they find uninspiring. While it is clear that faster brains and higher levels of energy do not always lead to more efficient processing of information, I wonder if we might find that further research shows that children with ADHD might display other types of “speed” that can be beneficial to them.